My first reaction was confusion, then panic. Everything had been going to plan: my brother Alistair and I were going full pelt in a five-man breakaway group on the bike, trying to take the race away from most of our rivals, when I saw a penalty board.

My first thought was: "Oh, No 31 – Alistair's got a penalty, what an idiot." Then I looked at my left arm and my right. The evidence was right there in front of me: I was 31.

I had never got a time penalty in a triathlon race before, and my mind was in overdrive. I haven't seen the replays yet, but I've been told my foot was on the line when I got on my bike in transition. Either way, there was nothing I could do about it. It's not like you can appeal to an official when you're going 30 miles an hour.

Alistair was brilliant though. There were only 15 seconds a lap on the bike where we could hear each other. He didn't realise I'd incurred a penalty, but when I told him, his first response was: "Calm down. You can still do this."

All sorts of calculations were going through my mind. OK, so I'm going to lose 15 seconds. What's that in terms of metres? Roughly 90m. OK. And I beat Javier Gomez in Kitzbuhel by over 30 seconds, so yes, Alistair is right, I can do this. I've just to run harder and faster than I've ever done before.

I was helped enormously by the biggest, noisiest and most raucous crowd I've ever raced in front of. They were amazing. They kept shouting "Go Alistair" or "Go Jonny". I was thinking: I'm trying, I'm trying!

Alistair really pushed it on the run. Afterwards, we found out he did the first 2.5km at 28-minutes-for-10,000m pace, straight off the bike when your legs are feeling wobbly. When you think that Mo Farah ran 27m 30sec to win the men's 10,000m on Saturday, you realise that we were going some.

The British coaches had told me to take my penalty at the end of the first lap of the run but I was in a breakaway group so I thought I'd wait. When it got to the third lap I was tiring so I thought I'd better take it. I quite appreciated the rest.

Would I have beaten Javier Gomez to the silver medal without the penalty? I don't think so. Even though the gap at the end was only 20 seconds, he looked strong throughout. But it will always be at the back of my mind.

After the finish, I started to feel worse. I was hot already, but then they took us to a tent to get us ready for the presentation which was like an oven. After about 10 minutes I collapsed. I'm not sure if I passed out or not, but I was certainly woozy. Then I was sick. A doctor came but it took a while and I was feeling awful. She felt my head and said: "You're really hot, you need to lie down immediately."

She then covered me in ice and wet towels to bring my temperature down. They then put me on a wheelchair, which I wasn't sure I needed, and took me to the medical room where they tested my blood pressure, heart rate and temperature and I was given glucose and water. After 20 minutes of having the ice on me, my temperature was exactly 38c, nothing really to worry about. It's super serious when it's over 40c. But I soon recovered and thankfully was declared well enough to collect my medal.

Being on the podium was fantastic and thrilling. But to be honest, I wasn't sure what to do. Should I smile? Should I sing? I did try a few words of the national anthem but I'm not someone who shows much emotion on the outside. But inside I was trying to absorb everything – the cheering crowds, the anthem, the sight of two British flags rising into the London sky. My internal monologue, though, was not much more than: wow, wow, wow, wow!

The strange thing about the race was how I didn't feel nervous at all.

This was the biggest race of my life, and normally beforehand I'm adjusting my kit and checking everything about five times to make sure everything is in place. I was almost expecting the Olympics to be the biggest thing ever, but it wasn't. It was just another race. I was more excited than nervous, and proud to be part of a team with Alistair and Stuart Hayes, who did a brilliant job on the bike – maintaining a high pace and protecting us from attacks.

How I'm going to celebrate? We only got to London on Saturday and we've been staying in Hyde Park so we've not experienced the village yet. But that will change. I'm also going to watch some races, and take it easy for a few days. But in two and a half weeks I'll be back on the treadmill with a world series race in Stockholm.

As for long-term ambitions, I'm moving out of Alistair's house and into my own place soon. And, longer term, I'm definitely looking at Rio 2016 if I still love the sport and all the training. I'll only be 26 then so hopefully I'll be in my prime. Who knows, maybe I can give Alistair a run for his money.